This invention relates to a process to produce molded articles of thermoplastic polymer reinforced with inorganic fiber and having excellent physical properties, especially high heat deflection temperature.
In conventional injection molding processes for making fiber-reinforced, thermoplastic polymer articles, raw material is supplied to the molding machine as pellets made by a precompounding process. The precompounded pellets are composed of thermoplastic polymer, reinforcement fiber and other ingredients including, for example, stabilizers, pigments, processing aids, modifiers and fillers. The precompounding step provides all the ingredients to the molding operation in proper proportion in convenient form and does not require handling them individually. It also disperses the ingredients for effectiveness in the article produced by the subsequent molding step. Precompounding does, however, add significantly to overall molded article production cost because it requires a second melting of the materials, isolation of pelletized intermediate product with attendant cost for yield loss and packaging, and usually a second drying operation before precompounded pellets are fed to the molding machine. Mixing during precompounding also causes fiber comminution to shorter length than was fed to the precompounding apparatus. Short fibers impart inferior physical properties, such as stiffness measured by flexural modulus and heat resistance measured by heat deflection temperature, than do long fibers. Because the precompounding step requires processing in the melt phase, matrix polymer is necessarily exposed to high temperatures and is subject to thermal degradation.
Sometimes polymer modifiers which are incompatible with the matrix polymer, such as toughening agents to improve impact resistance of the molded article, are added. Intensive mixing is required to disperse the agents in the matrix polymer. Unfortunately, intensive melt mixing also causes comminution of the reinforcement fibers.
If the ingredients of the composition could be fed to the molding machine directly, it would be possible to avoid the adverse performance effects and increased costs of operating the precompounding step. However, when chopped glass fiber bundles are conveyed and fed to injection molding machines together with un-precompounded thermoplastic polymer compositions, problems are often encountered. One problem is that the fiber bundles segregate from the pellet resin because of bulk density, and flow characteristics. Another problem is that the fiber bundles break apart during transport to the molding machine and produce fluffy agglomerates sometimes called "fuzzballs", which obstruct material flow and produce molded article defects. The result of these problems is that the molding process is frequently upset, dispersion of modifiers, reinforcement fiber and other ingredients in the matrix polymer is nonuniform and physical properties of the molded articles are inferior and inconsistent.
In U.S. Pat. No. 3,644,271, Tulley discloses an injection molding process in which the feeds are 20-40 wt % thermoplastic powder of 30-200 mesh size. 40-60 wt % thermoplastic pellets and about 20 wt % glass fibers. This process requires that a portion of the thermoplastic polymer is ground to a fine powder.
U.S. Pat. No. 3,732,345 to Amos et at. discloses alternative processes for incorporating filamentary reinforcing agent into thermoplastic resinous material. In one embodiment disclosed in the patent, chopped fibers and particulate resin are fed to the throat of the injection molding machine by being carefully deposited in separate layers on a conveyor. In other embodiments, the reinforcement agent and thermoplastic material are fed as separate streams to the molding machine throat. Amos does not explain how his methods overcome the above-mentioned problems.
In "Injection Molding of Carbon-Fiber Reinforced Polycarbonate" 0.2376 Kunststoffe 78 (1988), Kompalik et al. disclose an injection molding process using carbon fibers in polycarbonate resin as a two-component system. The fibers were specially pre-treated for adherence in polycarbonate.
It is desirable to provide a cost effective injection molding process for producing inorganic fiber-reinforced, thermoplastic polymer, consistent quality shaped articles having superior heat deflection temperature without having to resort to a precompounding step.